Measurement of the electrical resistivity of the earth is one of the most venerable geophysical methods (e.g. see Gish, O. H., and Rooney, W. J, “Measurements of large masses of undisturbed earth”, Terrestrial Magnetism, Vol. 30, No. 4, pp 161–188, 1925). Such measurements of resistivity are commonly made by passing electrical current of a selected waveform between two ground contact points, referred to as “current electrodes” and measuring the resultant voltages between two other ground contact points, referred to as “potential electrodes”. The geometrical disposition of the four electrodes, termed the “array”, may vary, depending on local circumstances and preferences of the survey operator. These arrays may be known by the names of their initial users, such as Schlumberger, Wenner, or by their description, such as “dipole-dipole”, or “pole-dipole”, etc.
Somewhat more recently, measurements of the induced polarization characteristics of the earth have been made using similar electrode arrays (e.g. see Seigel, H. O. “Mathematical formulation and type curves for induced polarization”, Geophysics Vol. 24, pp 547–563, 1959). There are other geoelectrical methods as well, all of which require making ground contact at multitudinous locations.
Typically, a large suite of such measurements are made, in a systematic fashion, over the area of interest, so that a map may be drawn up showing the distribution of the resistivity and/or induced polarisation characteristics, etc., over the surface of the survey area. Depending on the specific instrumentation employed, either single sequential measurements of these electrical properties may be made or, more efficiently, multiple concurrent measurements may be made, using multiple measuring circuits and multi-conductor cables.
To facilitate the making of a large number of measurements, either sequential or concurrent, it is common practice to use multi-conductor cables, each with a series of ground contact points (or take-outs), at intervals along the cable. For each individual measurement a selection of ground contact points has to be made, namely two for passing current into the ground (current electrodes) and two for measuring the resultant ground voltages (potential electrodes). Older instruments employed for this purpose utilized cables with as many conductors as there were take-outs, and with all of the conductors terminating at a switch box of an electronic console, at the position of the survey operator. In such instruments the operator carried out the selection of electrodes to be employed for a specific reading, either manually, or through software, at the switch box.
In order to reduce the number of individual conductors in the field cables, software controlled switches have been introduced at each take-out point along the cable. These switches are programmed for connecting the desired electrodes to either the desired current conductors or the desired potential (measuring) conductors, based on multiple software addresses. For example, U.S. Pat. No. 6,404,203, (M. S. B. Langmanson), discloses software-controlled contacts for creating the desired array geometry. This approach to software-controlled selective switching of electrodes in multi-electrode cables is utilized, for example, in the SARIS™, automated resistivity system of Scintrex, Limited, Concord Ontario, and in the Super-Sting™ resistivity instrument of Advanced Geoscience, Inc., Austin, Tex.
Typically, in these devices, each of the software-controlled take-outs has its own address code, and is activated to connect it to the local ground point (electrode) when it receives its address code, transmitted down the cable. The use of an address code, that is unique to each take-out position on the cable, facilitates efficiency in the coverage of large areas, but it has certain shortcomings as well. For example, when two sections of multi-take-out cables are connected together, for the so-called “roll-along” technique, special software is required to recognize that a take-out in the second cable is desired to be switched rather than a take-out at the identical position in the first cable. In addition, if one electrode switch becomes faulty, it must be replaced with one that has the identical address as the faulty switch, which makes the replacement more difficult.
It is a purpose of this invention to provide intelligent take-outs to ground that are, in all respects, identical, including their software code address, so that each electronic switch may act as a replacement for any faulty switch, regardless of its position along the cable. It is another purpose of this invention to provide an electronic switch that may be addressed from either end of the cable on which it lies, so that each cable may function equally well when oriented in either direction along a survey line.